1. Field of the Invention
The present invention relates to a wavelength division multiplexed (WDM) optical amplifier able to be used for an in-line amplifier, pre-amplifier, or post-amplifier in a WDM optical communications network.
2. Description of the Related Art
In recent years, optical communications networks have been demanded to be upgraded to enable higher multimedia services. For this, the amount of information transmitted has been growing rapidly. WDM optical amplifiers (hereinafter sometimes simply referred to as “optical amplifiers”) are essential and are broadly used to deal with this. In addition, it is demanded to further improve the optical amplifiers.
With the recent rapid growth in the market for metropolitan networks, further improvements are being sought from optical amplifiers. The main points for improvement are the fluctuation in output due to amplified spontaneous emission (ASE) and oscillation. As will be explained later with reference to the drawings, as general optical amplifiers, extensive use is being made of two-stage configurations comprised of a front-stage optical amplifying unit and a rear-stage optical amplifying unit. Due to this, optical amplifiers free of any wavelength dependency of the gain are being realized.
In such general two-stage configuration optical amplifiers, as explained later with reference to FIG. 1, the front-stage optical amplifying unit individually performs its own front-stage automatic gain control (AGC), while the rear-stage optical amplifying unit individually performs its own rear-stage AGC. Such a front-stage/rear-stage individual AGC type optical amplifier is disclosed for example in Japanese Unexamined Patent Publication (Kokai) No. 8-248455, Japanese Unexamined Patent Publication (Kokai) No. 2002-368698, and Japanese Unexamined Patent Publication (Kokai) No. 2003-174421.
This front-stage/rear-stage individual AGC, however, is not suitable for an optical amplifier for a metropolitan network. The reason is that high speed control is difficult for an optical amplifier operating based on individual two-stage AGC. For example, it is not possible to follow rapid changes in the number of input wavelengths, such as for example a change from 40 wavelengths to one wavelength, which occurs frequently in a metropolitan network.
Therefore, to cope with rapid fluctuations in the number of input wavelengths, recently, two-stage common AGC, instead of the above two-stage individual AGC, has been proposed for two-stage configuration optical amplifiers and has started being put into use. According to this two-stage common AGC type optical amplifier, however, while high speed control can be satisfied, the problems of fluctuation in output due to ASE and oscillation, explained in detail later, end up occurring and require improvement.